1. Field of the Invention
The present invention relates to an image recording apparatus such as a laser printer, a digital copying machine, digital facsimile or the like, and particularly to an image recording apparatus using multiple laser beams for writing an image.
2. Discussion of the Background
Conventionally, there is an image recording apparatus, such as a laser printer, having a plurality of laser diodes (hereinafter, LDs) used for high-speed recording by outputting light beams (e.g., laser beams) modulated according with image information from respective of LDs. The modulated light beams scan a record medium (e.g., a photosensitive body) by deflecting the light beams with a polygon mirror so that the image information can be recorded on the record medium.
In some cases, this type of image recording apparatus is required to write image information in different scanning densities (i.e., densities of picture elements).
Generally, while writing image information with a single beam, it is possible to write image information in different scanning densities in a single unit by changing appropriately a deflection speed of the light beam, a process speed, or a write frequency (i.e., image frequency).
In recent years, needs have arose for increased image recording speed or for upgraded image quality, and therefore the operation speed of the polygon mirror or the image frequency has desirably increased as well. However, the amount of increase in operational speed has a practical limit in either case due to technical difficulties. Accordingly, a multibeam system is often preferred over single-beam systems, as the multiple beams reduce a workload on system components when the operation speed of the polygon mirror or the image frequency is increased.
To change the scanning density in multibeam systems, however, a pitch (i.e., a beam pitch) in a subscanning direction of each light beam in a record medium must also be changed; hence there is a well-known image recording apparatus which can change the LD arrangement automatically so that the beam pitch becomes a pitch-according to the scanning density, for example, as described in Japanese Unexamined Patent Publication No. 56-104315.
As identified and addressed by the present invention, the conventional image recording apparatus has a problem that the beam pitch is sometimes affected by environmental conditions such as by temperature or humidity, and therefore an image quality is lowered if image information is recorded on a record medium by outputting a light beam modulated based on the image information from each LD and scanning the record medium by using the light beam under such conditions.